These days, several detection and analysis methods for determining physiological parameters in bodily fluid samples or in biological samples are carried out in large numbers in an automated manner in corresponding in vitro diagnostic systems. To this end, use is made of vessels referred to as cuvettes, which are suitable for samples, reagents and also the actual detection reaction. The blood samples are supplied to the device in blood sample tubes.
Blood sample tubes are usually manufactured from transparent plastic or glass and equipped at the tip thereof with a special connector for cannulas.
Current in vitro diagnostic systems are able to carry out a multiplicity of detection reactions and analyses using one sample. To this end, such devices usually comprise a receptacle position for a reaction vessel and an analysis system associated with the receptacle position. In order to be able to carry out a multiplicity of examinations in an automated manner, it is necessary to take small amounts of liquid from the corresponding containers by way of automated pipetting at a number of points. Thus, for example, aliquots of the blood sample must be taken from the blood sample tubes or exactly predetermined portions of reagents must be taken from the reagent containers and transferred into the reaction vessel provided for the respective examination. To this end, a plurality of appropriate pipetting systems are provided in the system, depending on usage purpose.
Such a pipetting system usually has a pipetting needle at an actively movable element, such as, e.g., a transport arm or swivel arm, which pipetting needle is fastened in a needle holder on the pipetting arm. The pipetting needle is configured as a hollow needle which is able to take defined amounts of sample in an automated manner under operation with pressure or negative pressure, with and without control liquids. The pipetting needle is inserted along the central axis of the respective vessel, penetrates an elastic sealing plug in the case of sealed vessels when necessary and is immersed into the liquid. The immersion, i.e., the contact with the liquid surface, is detected by means of an appropriate contact detection apparatus and the predetermined amount is sucked in under pressure control. The removed amount is then supplied to the appropriate analysis. Subsequently, the pipetting needle is rinsed in an appropriate apparatus and it is available for the next use.
In the above-described process, the reliable identification of contact with the surface of the liquid is mandatory in order, firstly, to determine the fill level of the liquid and, secondly, to ensure that no air is pipetted. What is problematic in this case is that the amounts of liquid to be detected are very small (in part a few μL), and so known techniques of fill level measurement, such, as e.g., by way of floats, inductivity or conductivity, are hardly usable.
Therefore, in the past, in vitro diagnostic systems have generally used a capacitive measurement, which was realized as purely analog switching technology on the basis of operational amplifiers. The necessary sensitivity could only be achieved by the detection of changes in capacitance, which were then reported to the control electronics by a voltage pulse.
A disadvantage here is that occurring disturbances, such as, e.g., electrostatic discharges and radio frequency fields, may likewise generate such voltage pulses. Therefore, there has been no possibility until now of distinguishing between disturbance and real voltage pulse as a contact signal in known systems.